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Effect of preparation method on particle properties of carbonate-type magnesium–aluminum layered double hydroxides
G Model JIEC 3381 No. of Pages 6
Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx
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Effect of preparation method on particle properties of carbonate-type magnesium–aluminum layered double hydroxides Tomohito Kameda* , Yoshiaki Umetsu Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 1,1 Katahira 2-Chome, Aoba-ku, Sendai 980-8577, Japan
A R T I C L E I N F O
Article history: Received 17 January 2017 Received in revised form 4 March 2017 Accepted 3 April 2017 Available online xxx Keywords: Magnesium–aluminum layered double hydroxide Preparation Particle size distribution Particle morphology Sedimentation
A B S T R A C T
Carbonate ion-intercalated Mg–Al layered double hydroxides (CO3-type Mg–Al LDHs) were prepared by using various methods to mix a solution of Mg(NO3)2 and Al(NO3)3 with an alkaline solution, and the particle properties of the obtained samples were compared. By mixing stoichiometric quantities of Mg2+, Al3+, and OH according to the coprecipitation reaction for preparing CO3-type Mg–Al LDHs, Mg2+ and Al3+ in solution were quantitatively precipitated, and the Mg/Al molar ratios of the obtained Mg–Al LDHs were equal to those of the solution, irrespective of the preparation method. However, the different preparation methods resulted in different particle properties, namely, different particle size distributions, particle morphologies, and sedimentation properties were observed. These differences were attributed to different formation processes for Mg–Al LDH. The ideal preparation method was determined to involve the addition of Mg(NO3)2 and Al(NO3)3 solution to Na2CO3 solution at a constant pH, which was achieved by adjusting with NaOH solution. This preparation method resulted in the formation of CO3-type Mg–Al LDH particles with uniform primary particles, good sedimentation properties, and a narrow distribution of secondary particle aggregates. Such characteristics make these Mg–Al LDHs excellent candidates for wastewater treatment. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Mg–Al layered double hydroxides (LDHs) are inorganic compounds with anion exchange properties [1–3]. Mg–Al LDHs are represented by the general formula [Mg2+1xAl3+x(OH)2](An)x/nmH2O, where x denotes the Al/(Mg + Al) molar ratio and An is an anion with a valency of n. Mg–Al LDHs consist of stacks of Al-bearing brucite-like octahedral layers, in which the positive charge owing to the replacement of some Mg with Al is electrically neutralized by interlayer anions. The interlayer space unoccupied by intercalated anions is occupied by water molecules. Mg–Al LDHs are prepared by mixing a solution of Mg2+ and Al3+ with an alkaline solution and then filtering the resultant suspension.
Abbreviations: LDH, layered double hydroxide; XRD, X-ray diffraction; ICP-AES, inductively coupled plasma-atomic emission spectrometry; SEM, scanning electron microscopy (SEM). * Corresponding author. Present address: Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan. Fax: +81 22 795 7212. E-mail address: [email protected] (T. Kameda).
Recently, Mg–Al LDHs have been examined for applications in wastewater treatment. For example, Mg–Al LDHs have the potential to remove selenate, selenite, and phosphate from aqueous solution owing to their anion exchange properties [4–7]. Moderately calcined Mg–Al LDH has been found to have potential for treating mineral acids [8–10]. Furthermore, Mg–Al LDHs intercalated with dodecylsulfate anions and ethylenediaminetetraacetate anions are capable of taking up bisphenol A and heavy metal ions from aqueous solution [11,12]. To apply Mg–Al LDHs for practical wastewater treatment, excellent solid–liquid separation properties are required. Typically, Mg–Al LDHs are obtained as suspensions of very small particles; therefore, solid–liquid separation requires a very long time owing to poor sedimentation and filtration properties. However, practical wastewater treatment is expected to need large amounts of Mg–Al LDH; accordingly, Mg–Al LDHs with rapid solid–liquid separation properties are required. Furthermore, as the Mg–Al LDH suspended in wastewater takes up hazardous materials, rapid filtration of the LDH particles from the wastewater is required. To realize these requirements, preparation of Mg–Al LDHs with larger particle sizes is desired. The preparation of Mg–Al LDHs by homogeneous precipitation methods using urea hydrolysis or a hydrothermal reaction has
http://dx.doi.org/10.1016/j.jiec.2017.04.009 1226-086X/© 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Kameda, Y. Umetsu, Effect of preparation method on particle properties of carbonate-type magnesium– aluminum layered double hydroxides, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.04.009
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been examined to control the particle size and morphology [13–16]. The purpose of these studies was to prepare monodisperse particles with large primary particle sizes. However, the hydrothermal method requires high temperature and pressure, and the urea method produces additional byproducts, such as NH4+. On the other hand, the very facile synthesis of nano-sized spherical Mg–Al LDH using an isoelectric point method has been developed recently [17]. The obtained nanospheres are very uniform, with an average size of 20 nm. Although this method is excellent, it also produces monodisperse particles with large primary particle sizes. In contrast, we have aimed to prepare Mg–Al LDH particles with excellent solid–liquid separation properties through growth of secondary particle aggregates at ambient temperature and pressure. To realize our purpose, it is necessary to first clarify the effect of preparation conditions and method on the particle properties of Mg–Al LDHs, such as particle size distribution, particle morphology, and sedimentation properties. In this study, carbonate ion-intercalated Mg–Al LDHs (CO3-type Mg–Al LDHs) were prepared by a coprecipitation reaction. Further, the effects of the NaOH concentration in the alkaline solution on the Mg/Al molar ratio of the LDH and the degrees of Mg2+ and Al3+ precipitated were examined. Thus, CO3-type Mg–Al LDHs were prepared by three different methods, and the particle properties of the obtained samples were compared. Moreover, the formation processes of the three types of LDHs were investigated. Although these results are mentioned briefly in our previous study [18], this paper presents these findings and associated data in detail.
Experimental All reagents were of chemical reagent grade (Wako Pure Chemical Industries, Ltd. (Japan)) and were used without further purification. Effect of NaOH concentration CO3-type Mg–Al LDH (Mg0.75Al0.25(OH)2(CO3)0.25) was obtained by dropwise addition of a Mg–Al solution to a NaOH/Na2CO3 solution according to the coprecipitation reaction expressed by Eq. (1). 0.75Mg2+ + 0.25Al3+ + 2OH + 0.125CO32 ! Mg0.75Al0.25(OH)2(CO3)0.125 (1) The Mg–Al solution (0.375 M Mg(NO3)2 + 0.125 M Al(NO3)3) was prepared by dissolving Mg(NO3)26H2O (93.75 mmol) and Al (NO3)39H2O (31.25 mmol) in 250 mL of deionized water. Four types of NaOH/Na2CO3 solution were prepared by dissolving the required amounts of NaOH and Na2CO3 in 250 mL of deionized water. The NaOH concentrations in the prepared solutions were 0.75, 0.9, 1.0, and 1.25 M, i.e., 0.75, 0.9, 1.0, and 1.25 times the required stoichiometric quantity, according to Eq. (1), whereas the Na2CO3 concentration in the four solutions was 0.125 M, i.e., 2.0 times the required stoichiometric quantity. The Mg–Al solution was added to the NaOH/Na2CO3 solution at a rate of 10 mL/min at 30 C under mild agitation. After addition of the Mg–Al solution, the resultant suspension was left to stand at 30 C for 1 h. The precipitates were recovered by filtering the resultant suspensions, washing them repeatedly with deionized water, and then drying them at 80 C for 40 h. The solution pH was continuously measured throughout the operation to monitor the mixing conditions. The precipitates were identified by X-ray diffraction (XRD) with CuKa radiation. The precipitates were also dissolved in 1 M HNO3, and the Mg2+ and Al3+ contents were analyzed using inductively coupled plasma-atomic emission
spectrometry (ICP-AES). Furthermore, the residual concentrations of Mg2+ and Al3+ in the filtrates were also determined by ICP-AES. Effect of preparation method Three methods for preparing Mg–Al LDHs were examined. The NaOH/Na2CO3 solution had NaOH and Na2CO3 concentrations of 1.0 and 0.125 M, respectively. Preparation (i): Addition of Mg–Al solution to NaOH/Na2CO3 solution This method is the same as that described in Section “Effect of NaOH concentration”. Preparation (ii): Addition of NaOH/Na2CO3 solution to Mg–Al solution The NaOH/Na2CO3 solution was added to the Mg–Al solution at a rate of 10 mL/min at 30 C under mild agitation. Preparation (iii): Addition of Mg–Al solution to Na2CO3 solution at constant pH A Na2CO3 solution (0.2 M Na2CO3) was prepared by dissolving Na2CO3 (30 mmol) in 150 mL of deionized water. This concentration of Na2CO3 corresponds to 2.0 times the stoichiometric quantity defined by Eq. (1). The Mg–Al solution was added to the Na2CO3 solution at a rate of 10 mL/min at 30 C under mild agitation, and the solution pH was adjusted to 11.0 by addition of 2.5 M NaOH solution. The mixed Mg–Al and alkaline solutions in preparations (i)–(iii) were treated as described in Section “Effect of NaOH concentration”, and the obtained precipitates and filtrates were analyzed. Furthermore, the resultant suspensions were dispersed in ethanol, and the particle size distributions were determined using a laser particle size analyzer. The resultant suspensions were also centrifuged, repeatedly washed with deionized water, and then freeze-dried for about 24 h. The morphologies of the freeze-dried samples were examined using scanning electron microscopy (SEM). Dilute suspensions were prepared using one twentieth of the concentrations of the starting solution in preparations (i)–(iii) to examine the sedimentation properties of the Mg–Al LDH particles. The dilute suspensions (400 mL) were added to a graduated cylinder, and variations in the height of the sediment with time were measured. As the particle properties of the Mg–Al LDHs obtained using preparations (i)–(iii) are considered to greatly depend on the formation processes, these processes of were examined by characterizing the products at several steps during the addition of Mg–Al solution or NaOH/Na2CO3 solution. For preparation (i) or (iii), 50–250 mL of Mg–Al solution was added to 250 mL of NaOH/ Na2CO3 solution or 150 mL of Na2CO3 solution, respectively. For preparation (ii), 50–250 mL of NaOH/Na2CO3 solution was added to 250 mL of Mg–Al solution. The mixed solutions were treated as described in Section “Effect of NaOH concentration”, and the obtained precipitates and filtrates were analyzed. Results and discussion Effect of NaOH concentration Mg–Al LDHs are usually prepared by mixing a solution of Mg2+ and Al3+ with an alkaline solution, such as NaOH solution. The prepared precipitates of Mg–Al LDH are filtered, and then washed repeatedly with water until the filtrate becomes neutral. If the NaOH concentration is low, some Mg2+ is not precipitated as Mg–Al LDH and remains in the filtrate, which causes high amounts of Mg2+ to be wasted in industrial preparation. If the NaOH concentration is high, large volumes of water are required for washing to remove alkaline material attached to the precipitates.
Please cite this article in press as: T. Kameda, Y. Umetsu, Effect of preparation method on particle properties of carbonate-type magnesium– aluminum layered double hydroxides, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.04.009
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4.0
3.0 50 2.0
1.0
0 0.5
Mg/Al mole ratio ( ) in precipitate
5.0
100 Mg2+ ( ) and Al3+ ( ) precipitated / %
Therefore, the optimum NaOH concentration for the preparation of Mg–Al LDHs was examined. The precipitates obtained by adding the Mg–Al solution to NaOH/Na2CO3 solutions with various NaOH concentrations were identified by XRD, as shown in Fig. 1. All the XRD peaks could be indexed to hydrotalcite (JCPDS card 22-700), a hydroxycarbonate of Mg and Al formulated as Mg6Al2(OH)16CO34H2O, which occurs in nature. This finding suggests that the precipitates are CO3-type Mg–Al LDHs. The XRD peaks in spectra (c) and (d) are sharper than those in spectra (a) and (b); furthermore, the peak intensities in (c) and (d) are higher than those in (a) and (b). Thus, the precipitate exhibited good crystallinity when the NaOH concentration was high. Fig. 2 presents the effect of NaOH concentration on the degree of Mg2+ and Al3+ precipitated and the Mg/Al molar ratio in the precipitates. The amount of precipitated Mg2+ increased with increasing NaOH concentration, reaching over 99% above 1.0 M, whereas more than 99% of Al3+ was precipitated in all cases. As a result, the Mg/Al molar ratio in the precipitate increased as the NaOH concentration increased. At NaOH concentrations over 1.0 M NaOH, the Mg/Al molar ratio in the precipitate was equal to that of the Mg–Al solution. The Mg–Al solution was added to the NaOH/Na2CO3 solution over 25 min, and then the mixed solution was agitated for 1 h. The variations in the solution pH were measured every 5 min during the addition of the Mg–Al solution and after the agitation, as presented in Table 1. With 0.75 and 0.9 M NaOH, the pH values after agitation were 7.9 and 9.1, respectively. From the potential–pH equilibrium diagram for the magnesium/water system [19], Mg (OH)2 is precipitated from about 0.375 M Mg2+ at pH 9.0. Therefore, the Mg2+ remaining in the filtrates for 0.75 and 0.9 M NaOH, as shown in Fig. 2, is attributed to a shortage of NaOH. In contrast, Al3+ is an amphoteric ion, and all pH values after agitation were within the range for formation of aluminum hydroxide [19], which is in accordance with the observation of more than 99% precipitated Al3+ in all cases. As shown in Fig. 1, the precipitates prepared from 0.75 and 0.9 M NaOH were found to have worse crystallinity than those prepared from 1.0 and 1.25 M NaOH. As shown in Table 1, all the precipitates were formed at very high pH values of approximately 13 until 15 min. However, the pH values decreased to 7.6 and 9.0 at 25 min for 0.75 and 0.9 M NaOH, respectively. A wide pH range during preparation is considered to result in the formation of heterogeneous Mg–Al LDH crystals. Furthermore, some precipitates initially formed at very high pH values may dissolve during
3
0 1.5
1.0 NaOH concentration / M
Fig. 2. Effects of NaOH concentration on the degrees of Mg2+ and Al3+ precipitated and the Mg/Al molar ratio in the precipitates.
Table 1 Variations in solution pH every 5 min during addition of Mg–Al solution and after agitation. (pH) After the agitation
NaOH/M
Addition time / min 0
5
10
15
20
25
0.75 0.9 1.0 1.25
13.4 13.5 13.6 13.6
13.2 13.3 13.5 13.5
13.0 13.1 13.3 13.3
12.6 12.9 13.1 13.1
10.4 12.4 12.7 12.8
7.6 9.0 10.8 12.6
7.9 9.1 11.0 12.6
agitation for 1 h at lower pH values. These two reasons probably lead to the low crystallinities of Mg–Al LDHs prepared from 0.75 and 0.9 M NaOH. The good crystallinities of CO3-type Mg–Al LDHs obtained over 1.0 M NaOH result from almost complete precipitation of Mg2+ and Al3+ in solution. However, the pH after agitation for 1.25 M NaOH was very high at 12.6. Therefore, the optimum NaOH concentration for Mg–Al LDH preparation by coprecipitation is 1.0 M. This stoichiometric addition of NaOH according to Eq. (1) results in precipitation of more than 99% of Mg2+ and Al3+ to give CO3-type Mg–Al LDH with a Mg/Al molar ratio of 3.0, equal to that of the Mg–Al solution.
110 113
018
009,012 015
006
003
Hydrotalcite
Relative Intensity
2000 cps
Effect of preparation method
(a)
(b)
(c)
(d) 10
20
30 40 50 2θ/ deg.(CuKα)
60
70
Fig. 1. XRD patterns of precipitates prepared from NaOH/Na2CO3 solutions with various NaOH concentrations: (a) 0.75, (b) 0.9, (c) 1.0, and (d) 1.25 M NaOH.
The effect of the preparation method on the particle properties of Mg–Al LDHs was examined. For preparation (i) or (iii), the Mg–Al solution was added to the NaOH/Na2CO3 solution or Na2CO3 solution, respectively, at constant pH. For preparation (ii), the NaOH/Na2CO3 solution was added to the Mg–Al solution. For preparations (i) and (ii), the Mg–Al and NaOH/Na2CO3 solutions were mixed at stoichiometric quantities, according to Eq. (1). For preparation (iii), the pH was adjusted to 11.0 to standardize the preparation conditions. This pH was chosen to coincide with the pH after agitation for preparation (i), as shown in Table 1. The variations in solution pH with time during the various preparations are presented in Fig. 3. As shown, the pH profiles of the three preparation methods are very different; thus, we examined the effect of these different routes on the particle properties of the prepared Mg–Al LDHs. The XRD patterns of the precipitates obtained using the three preparations confirmed the formation of CO3-type Mg–Al LDHs with Mg/Al molar ratios of 3.0. Although all the preparation methods resulted in the formation of CO3-type Mg–Al LDHs with
Please cite this article in press as: T. Kameda, Y. Umetsu, Effect of preparation method on particle properties of carbonate-type magnesium– aluminum layered double hydroxides, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.04.009
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14 the preparation (i)
12
the preparation (iii)
10
pH
8 the preparation (ii)
6 4 2 0
0
5
10 15 Addition time / min
20
25
Fig. 3. Variations in the solution pH over time during the various preparations.
the same Mg/Al molar ratios, the particle properties were found to be quite different. Fig. 4 shows SEM images of the freeze-dried samples. For preparation (i), very small particles of less than 0.1 mm in diameter were observed. Preparation (ii) produced secondary particle aggregates of 0.1–0.2 mm in diameter, which consisted of various sizes of plate-like crystals, whereas preparation (iii) produced secondary particle aggregates of 0.2–0.5 mm in diameter, with plate-like crystals of similar sizes. The particle size distributions in the suspensions resulting from the various preparations are presented in Fig. 5. The size distributions were 1–20 mm for preparation (i), 1–100 mm for (ii), and 2–20 mm for (iii), although the secondary particle sizes observed for the precipitates in the SEM images in Fig. 4 were less than 5 mm for all preparations. This finding supports the suggestion that CO3-type Mg–Al LDHs consist of secondary particle aggregates, and the different preparation methods were found to produce aggregates with different sizes. Furthermore, the LDH particles prepared by the different preparation methods exhibited different sedimentation properties. Fig. 6 shows the variations in the heights of the sediment over time for dilute suspensions. The suspensions displayed rapid sedimentation in the order (iii) > (ii) > (i). Preparation (iii) was found to produce CO3-type Mg–Al LDH particles with good solid–liquid separation properties. This finding corresponds to the very short time required for filtration of the LDH suspension in preparation (iii). To determine the origin of the different particle size distributions, particle morphologies, and sedimentation properties in the different preparation methods, the formation processes of Mg–Al LDHs were examined by characterizing the intermediate products at several steps during addition of Mg–Al solution or NaOH/Na2CO3 solution. When various volumes of Mg–Al solution (50, 100, 150, 200, and 250 mL) were added to 250 mL of the NaOH/Na2CO3 solution or 150 mL of the Na2CO3 solution in preparation (i) or (iii), respectively, CO3-type Mg–Al LDHs were precipitated with Mg/Al molar ratios of 3.0 at all volumes of the Mg–Al solution, and the degrees of Mg2+ and Al3+ precipitated were over 99%. Based on these results, CO3-type Mg–Al LDHs are likely precipitated throughout the addition time for preparations (i) and (iii) in Fig. 3. In preparation (i), Al(OH)4 is likely produced by the addition of the Mg–Al solution to the NaOH/Na2CO3 solution. Subsequently, Al(OH)4 reacts with Mg2+, resulting in the formation of Mg–Al LDH. For preparation (iii), to adjust the pH, a NaOH solution was added with the Mg–Al solution to the Na2CO3
Fig. 4. SEM images of freeze-dried samples: (a) preparation (i), (b) preparation (ii), and (c) preparation (iii).
solution. At the point where droplets of NaOH solution are added, the local pH will be very high. Owing to this high local pH, Al(OH)4 is produced from the added Mg–Al solution, and Mg–Al LDH is likely formed by reaction between Al(OH)4 and Mg2+. In contrast, some intermediate products were observed for preparation (ii). Fig. 7 shows XRD patterns for the precipitates obtained by addition of various volumes of NaOH/Na2CO3 solution to the Mg–Al solution for preparation (ii). The effect of the volume of NaOH/Na2CO3 solution on the degrees of Mg2+ and Al3+ precipitated and the Mg/Al molar ratio in the precipitate for preparation (ii) is presented in Fig. 8, and Table 2 gives the variations in solution pH with increasing volumes of NaOH/Na2CO3 solution. For 50 mL of the NaOH/Na2CO3 solution, neither Mg2+ nor Al3+ was precipitated owing to the low solution pH. For 75 mL, over 99% of Al3+ was
Please cite this article in press as: T. Kameda, Y. Umetsu, Effect of preparation method on particle properties of carbonate-type magnesium– aluminum layered double hydroxides, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.04.009
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15
15
15 ( b)
0 0
1 10 100 Particle size / μm
Distribution / %
5
5
0 0
( c)
10
10
Distribution / %
Distribution / %
(a)
10
5
5
0 0
1 10 100 Particle size / μm
1
10
100
Particle size / μm
Fig. 5. Particle size distribution profiles for the resultant suspensions: (a) preparation (i), (b) preparation (ii), and (c) preparation (iii).
300
200 (a) (b)
100
4.0
3.0 50
(c) 0 0
2.0
1.0
0
20
40
60 80 Time / min
100
0
100
150
0 250
200
Volume of NaOH-Na2CO3 solution / ml
120
Fig. 6. Variations in the height of sediment with time for dilute suspensions of (a) preparation (i), (b) preparation (ii), and (c) preparation (iii).
50
Mg/Al mole ratio ( ) in precipitate
5.0
100 Mg2+ ( ) and Al3+ ( ) precipitated / %
Height of sedimentation / ml
400
Fig. 8. Effects of volume of NaOH/Na2CO3 solution on the degrees of Mg2+ and Al3+ precipitated and the Mg/Al molar ratio in the precipitate for preparation (ii).
Table 2 Variations in the solution pH with increasing volume of NaOH/Na2CO3 solution.
2000 cps
Hydrotalcite
Al(OH)3 (a) (b)
Volume of NaOH–Na2CO3 solution/mL
50
75
100
150
200
(pH) 250
After the addition After the agitation
3.9 3.9
4.9 5.9
7.9 7.3
8.2 7.5
9.0 8.1
10.8 10.7
Relative Intensity
Initial pH: 2.7.
(c) (d) (e) 10
20
30 40 50 2θ/ deg.(CuKα)
60
70
Fig. 7. XRD patterns for precipitates obtained by the addition of various volumes of the NaOH/Na2CO3 solution to the Mg–Al solution for preparation (ii): (a) 75, (b) 100, (c) 150, (d) 200, and (e) 250 mL.
precipitated, whereas Mg2+ was not precipitated, which results in the formation of amorphous Al(OH)3. For 100 mL, both Mg–Al LDH and Al(OH)3 were observed in the XRD spectra, although the
degree of Mg2+ precipitated was very low. It was found that the Mg/Al LDH started to be formed at around pH 8. From the XRD spectra at over 150 mL, the precipitate was observed to contain only Mg–Al LDH. The degree of Mg2+ precipitated and the Mg/Al molar ratio in the precipitate increased with increasing volume of NaOH/Na2CO3 solution, and Mg–Al LDH with a Mg/Al molar ratio of 3.0 was precipitated with 250 mL of NaOH/Na2CO3 solution. Based on these results, in preparation (ii), Mg–Al LDH is considered to form through intermediate products. First, Al(OH)3 is likely produced by the addition of the NaOH/Na2CO3 solution to the Mg–Al solution. With increasing volumes of NaOH/Na2CO3 solution, Al(OH)4 probably forms by dissolution of Al(OH)3 owing to the high local pH caused by addition of the mixed alkaline solution. Then, Al(OH)4 reacts with Mg2+, leading to the formation
Please cite this article in press as: T. Kameda, Y. Umetsu, Effect of preparation method on particle properties of carbonate-type magnesium– aluminum layered double hydroxides, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.04.009
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of Mg–Al LDH. The gradual increase of pH from 2.5 to 4.5 observed in Fig. 3 for preparation (ii) is attributed to the consumption of OH by the formation of Al(OH)3. The second increase of pH from 8.0 to 11.0 is attributed to the consumption of OH by the formation of Mg–Al LDH. Based on these formation processes for Mg–Al LDHs, the different Mg–Al LDH particle properties obtained with the different preparation methods are considered as follows. The very small particles obtained using preparation (i) (Fig. 4(a)) are attributed to the generation of many Mg–Al LDH crystal nuclei owing to the very high pH, which do not show considerable growth. For preparation (ii) (Fig. 4(b)), various sizes of primary plate-like crystals are observed. In this case, the Mg–Al LDH crystal nuclei are considered to grow due to the low pH during preparation (pH 8.0–11.0, shown in Fig. 3), and this wide pH range probably results in the nonuniformity of the crystal sizes owing to variations during growth of the Mg–Al LDH crystal nuclei. In contrast, preparation (iii) produces plate-like crystals of similar sizes, as shown in Fig. 4(c). The constant pH leads to uniform growth of the Mg–Al LDH crystal nuclei. As shown in Fig. 5, the particle size distribution for preparation (i) is similar to that for preparation (iii), which is related to the similar formation processes for Mg–Al LDHs. However, preparation (ii) provides a wider distribution of particle sizes than preparations (i) and (iii). The first stage of preparation (ii) is considered to generate various sizes of aggregates of highly polymerized Al(OH)3 owing to repeated hydrolysis and olation. The transformation of this polymerized Al(OH)3 into Mg–Al LDH probably causes the wide particle size distribution. As presented in Fig. 6, sedimentation of the dilute suspension for preparation (iii) was faster than that for preparations (i) and (ii). The slow sedimentation for preparation (i) is attributed to the very small particles prepared. For preparation (ii), gelatinous Al(OH)3 is first produced as an intermediate product, likely resulting in the formation of gelatinous Mg–Al LDH, which could cause slow sedimentation. From these results, preparation (iii) is considered superior to preparations (i) and (ii) because it provides Mg–Al LDH with uniform primary particles, good sedimentation properties, and a narrow distribution of secondary particle aggregates, which are manageable. Hereafter, it will be necessary to develop preparation methods for Mg–Al LDH on the industrial scale by adapting the preparation (iii) method.
Conclusions In this study, three preparation methods with different pH profiles were examined for preparing CO3-type Mg–Al LDHs. Although all three methods provided Mg–Al LDHs with the same Mg/Al ratio, the particle properties were found to be significantly different owing to different formation processes. Overall, preparation (iii), in which a Mg–Al solution was added to a solution of Na2CO3 at constant pH, provided uniform crystalline LDH particles with a narrow distribution of secondary particle aggregates and excellent sedimentation properties. Scaling this method up to the industrial scale will allow sufficient particles to be produced for use in wastewater treatment. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jiec.2017.04.009. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19]
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Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx
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Effect of preparation method on particle properties of carbonate-type magnesium–aluminum layered double hydroxides Tomohito Kameda* , Yoshiaki Umetsu Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 1,1 Katahira 2-Chome, Aoba-ku, Sendai 980-8577, Japan
A R T I C L E I N F O
Article history: Received 17 January 2017 Received in revised form 4 March 2017 Accepted 3 April 2017 Available online xxx Keywords: Magnesium–aluminum layered double hydroxide Preparation Particle size distribution Particle morphology Sedimentation
A B S T R A C T
Carbonate ion-intercalated Mg–Al layered double hydroxides (CO3-type Mg–Al LDHs) were prepared by using various methods to mix a solution of Mg(NO3)2 and Al(NO3)3 with an alkaline solution, and the particle properties of the obtained samples were compared. By mixing stoichiometric quantities of Mg2+, Al3+, and OH according to the coprecipitation reaction for preparing CO3-type Mg–Al LDHs, Mg2+ and Al3+ in solution were quantitatively precipitated, and the Mg/Al molar ratios of the obtained Mg–Al LDHs were equal to those of the solution, irrespective of the preparation method. However, the different preparation methods resulted in different particle properties, namely, different particle size distributions, particle morphologies, and sedimentation properties were observed. These differences were attributed to different formation processes for Mg–Al LDH. The ideal preparation method was determined to involve the addition of Mg(NO3)2 and Al(NO3)3 solution to Na2CO3 solution at a constant pH, which was achieved by adjusting with NaOH solution. This preparation method resulted in the formation of CO3-type Mg–Al LDH particles with uniform primary particles, good sedimentation properties, and a narrow distribution of secondary particle aggregates. Such characteristics make these Mg–Al LDHs excellent candidates for wastewater treatment. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Mg–Al layered double hydroxides (LDHs) are inorganic compounds with anion exchange properties [1–3]. Mg–Al LDHs are represented by the general formula [Mg2+1xAl3+x(OH)2](An)x/nmH2O, where x denotes the Al/(Mg + Al) molar ratio and An is an anion with a valency of n. Mg–Al LDHs consist of stacks of Al-bearing brucite-like octahedral layers, in which the positive charge owing to the replacement of some Mg with Al is electrically neutralized by interlayer anions. The interlayer space unoccupied by intercalated anions is occupied by water molecules. Mg–Al LDHs are prepared by mixing a solution of Mg2+ and Al3+ with an alkaline solution and then filtering the resultant suspension.
Abbreviations: LDH, layered double hydroxide; XRD, X-ray diffraction; ICP-AES, inductively coupled plasma-atomic emission spectrometry; SEM, scanning electron microscopy (SEM). * Corresponding author. Present address: Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan. Fax: +81 22 795 7212. E-mail address: [email protected] (T. Kameda).
Recently, Mg–Al LDHs have been examined for applications in wastewater treatment. For example, Mg–Al LDHs have the potential to remove selenate, selenite, and phosphate from aqueous solution owing to their anion exchange properties [4–7]. Moderately calcined Mg–Al LDH has been found to have potential for treating mineral acids [8–10]. Furthermore, Mg–Al LDHs intercalated with dodecylsulfate anions and ethylenediaminetetraacetate anions are capable of taking up bisphenol A and heavy metal ions from aqueous solution [11,12]. To apply Mg–Al LDHs for practical wastewater treatment, excellent solid–liquid separation properties are required. Typically, Mg–Al LDHs are obtained as suspensions of very small particles; therefore, solid–liquid separation requires a very long time owing to poor sedimentation and filtration properties. However, practical wastewater treatment is expected to need large amounts of Mg–Al LDH; accordingly, Mg–Al LDHs with rapid solid–liquid separation properties are required. Furthermore, as the Mg–Al LDH suspended in wastewater takes up hazardous materials, rapid filtration of the LDH particles from the wastewater is required. To realize these requirements, preparation of Mg–Al LDHs with larger particle sizes is desired. The preparation of Mg–Al LDHs by homogeneous precipitation methods using urea hydrolysis or a hydrothermal reaction has
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been examined to control the particle size and morphology [13–16]. The purpose of these studies was to prepare monodisperse particles with large primary particle sizes. However, the hydrothermal method requires high temperature and pressure, and the urea method produces additional byproducts, such as NH4+. On the other hand, the very facile synthesis of nano-sized spherical Mg–Al LDH using an isoelectric point method has been developed recently [17]. The obtained nanospheres are very uniform, with an average size of 20 nm. Although this method is excellent, it also produces monodisperse particles with large primary particle sizes. In contrast, we have aimed to prepare Mg–Al LDH particles with excellent solid–liquid separation properties through growth of secondary particle aggregates at ambient temperature and pressure. To realize our purpose, it is necessary to first clarify the effect of preparation conditions and method on the particle properties of Mg–Al LDHs, such as particle size distribution, particle morphology, and sedimentation properties. In this study, carbonate ion-intercalated Mg–Al LDHs (CO3-type Mg–Al LDHs) were prepared by a coprecipitation reaction. Further, the effects of the NaOH concentration in the alkaline solution on the Mg/Al molar ratio of the LDH and the degrees of Mg2+ and Al3+ precipitated were examined. Thus, CO3-type Mg–Al LDHs were prepared by three different methods, and the particle properties of the obtained samples were compared. Moreover, the formation processes of the three types of LDHs were investigated. Although these results are mentioned briefly in our previous study [18], this paper presents these findings and associated data in detail.
Experimental All reagents were of chemical reagent grade (Wako Pure Chemical Industries, Ltd. (Japan)) and were used without further purification. Effect of NaOH concentration CO3-type Mg–Al LDH (Mg0.75Al0.25(OH)2(CO3)0.25) was obtained by dropwise addition of a Mg–Al solution to a NaOH/Na2CO3 solution according to the coprecipitation reaction expressed by Eq. (1). 0.75Mg2+ + 0.25Al3+ + 2OH + 0.125CO32 ! Mg0.75Al0.25(OH)2(CO3)0.125 (1) The Mg–Al solution (0.375 M Mg(NO3)2 + 0.125 M Al(NO3)3) was prepared by dissolving Mg(NO3)26H2O (93.75 mmol) and Al (NO3)39H2O (31.25 mmol) in 250 mL of deionized water. Four types of NaOH/Na2CO3 solution were prepared by dissolving the required amounts of NaOH and Na2CO3 in 250 mL of deionized water. The NaOH concentrations in the prepared solutions were 0.75, 0.9, 1.0, and 1.25 M, i.e., 0.75, 0.9, 1.0, and 1.25 times the required stoichiometric quantity, according to Eq. (1), whereas the Na2CO3 concentration in the four solutions was 0.125 M, i.e., 2.0 times the required stoichiometric quantity. The Mg–Al solution was added to the NaOH/Na2CO3 solution at a rate of 10 mL/min at 30 C under mild agitation. After addition of the Mg–Al solution, the resultant suspension was left to stand at 30 C for 1 h. The precipitates were recovered by filtering the resultant suspensions, washing them repeatedly with deionized water, and then drying them at 80 C for 40 h. The solution pH was continuously measured throughout the operation to monitor the mixing conditions. The precipitates were identified by X-ray diffraction (XRD) with CuKa radiation. The precipitates were also dissolved in 1 M HNO3, and the Mg2+ and Al3+ contents were analyzed using inductively coupled plasma-atomic emission
spectrometry (ICP-AES). Furthermore, the residual concentrations of Mg2+ and Al3+ in the filtrates were also determined by ICP-AES. Effect of preparation method Three methods for preparing Mg–Al LDHs were examined. The NaOH/Na2CO3 solution had NaOH and Na2CO3 concentrations of 1.0 and 0.125 M, respectively. Preparation (i): Addition of Mg–Al solution to NaOH/Na2CO3 solution This method is the same as that described in Section “Effect of NaOH concentration”. Preparation (ii): Addition of NaOH/Na2CO3 solution to Mg–Al solution The NaOH/Na2CO3 solution was added to the Mg–Al solution at a rate of 10 mL/min at 30 C under mild agitation. Preparation (iii): Addition of Mg–Al solution to Na2CO3 solution at constant pH A Na2CO3 solution (0.2 M Na2CO3) was prepared by dissolving Na2CO3 (30 mmol) in 150 mL of deionized water. This concentration of Na2CO3 corresponds to 2.0 times the stoichiometric quantity defined by Eq. (1). The Mg–Al solution was added to the Na2CO3 solution at a rate of 10 mL/min at 30 C under mild agitation, and the solution pH was adjusted to 11.0 by addition of 2.5 M NaOH solution. The mixed Mg–Al and alkaline solutions in preparations (i)–(iii) were treated as described in Section “Effect of NaOH concentration”, and the obtained precipitates and filtrates were analyzed. Furthermore, the resultant suspensions were dispersed in ethanol, and the particle size distributions were determined using a laser particle size analyzer. The resultant suspensions were also centrifuged, repeatedly washed with deionized water, and then freeze-dried for about 24 h. The morphologies of the freeze-dried samples were examined using scanning electron microscopy (SEM). Dilute suspensions were prepared using one twentieth of the concentrations of the starting solution in preparations (i)–(iii) to examine the sedimentation properties of the Mg–Al LDH particles. The dilute suspensions (400 mL) were added to a graduated cylinder, and variations in the height of the sediment with time were measured. As the particle properties of the Mg–Al LDHs obtained using preparations (i)–(iii) are considered to greatly depend on the formation processes, these processes of were examined by characterizing the products at several steps during the addition of Mg–Al solution or NaOH/Na2CO3 solution. For preparation (i) or (iii), 50–250 mL of Mg–Al solution was added to 250 mL of NaOH/ Na2CO3 solution or 150 mL of Na2CO3 solution, respectively. For preparation (ii), 50–250 mL of NaOH/Na2CO3 solution was added to 250 mL of Mg–Al solution. The mixed solutions were treated as described in Section “Effect of NaOH concentration”, and the obtained precipitates and filtrates were analyzed. Results and discussion Effect of NaOH concentration Mg–Al LDHs are usually prepared by mixing a solution of Mg2+ and Al3+ with an alkaline solution, such as NaOH solution. The prepared precipitates of Mg–Al LDH are filtered, and then washed repeatedly with water until the filtrate becomes neutral. If the NaOH concentration is low, some Mg2+ is not precipitated as Mg–Al LDH and remains in the filtrate, which causes high amounts of Mg2+ to be wasted in industrial preparation. If the NaOH concentration is high, large volumes of water are required for washing to remove alkaline material attached to the precipitates.
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4.0
3.0 50 2.0
1.0
0 0.5
Mg/Al mole ratio ( ) in precipitate
5.0
100 Mg2+ ( ) and Al3+ ( ) precipitated / %
Therefore, the optimum NaOH concentration for the preparation of Mg–Al LDHs was examined. The precipitates obtained by adding the Mg–Al solution to NaOH/Na2CO3 solutions with various NaOH concentrations were identified by XRD, as shown in Fig. 1. All the XRD peaks could be indexed to hydrotalcite (JCPDS card 22-700), a hydroxycarbonate of Mg and Al formulated as Mg6Al2(OH)16CO34H2O, which occurs in nature. This finding suggests that the precipitates are CO3-type Mg–Al LDHs. The XRD peaks in spectra (c) and (d) are sharper than those in spectra (a) and (b); furthermore, the peak intensities in (c) and (d) are higher than those in (a) and (b). Thus, the precipitate exhibited good crystallinity when the NaOH concentration was high. Fig. 2 presents the effect of NaOH concentration on the degree of Mg2+ and Al3+ precipitated and the Mg/Al molar ratio in the precipitates. The amount of precipitated Mg2+ increased with increasing NaOH concentration, reaching over 99% above 1.0 M, whereas more than 99% of Al3+ was precipitated in all cases. As a result, the Mg/Al molar ratio in the precipitate increased as the NaOH concentration increased. At NaOH concentrations over 1.0 M NaOH, the Mg/Al molar ratio in the precipitate was equal to that of the Mg–Al solution. The Mg–Al solution was added to the NaOH/Na2CO3 solution over 25 min, and then the mixed solution was agitated for 1 h. The variations in the solution pH were measured every 5 min during the addition of the Mg–Al solution and after the agitation, as presented in Table 1. With 0.75 and 0.9 M NaOH, the pH values after agitation were 7.9 and 9.1, respectively. From the potential–pH equilibrium diagram for the magnesium/water system [19], Mg (OH)2 is precipitated from about 0.375 M Mg2+ at pH 9.0. Therefore, the Mg2+ remaining in the filtrates for 0.75 and 0.9 M NaOH, as shown in Fig. 2, is attributed to a shortage of NaOH. In contrast, Al3+ is an amphoteric ion, and all pH values after agitation were within the range for formation of aluminum hydroxide [19], which is in accordance with the observation of more than 99% precipitated Al3+ in all cases. As shown in Fig. 1, the precipitates prepared from 0.75 and 0.9 M NaOH were found to have worse crystallinity than those prepared from 1.0 and 1.25 M NaOH. As shown in Table 1, all the precipitates were formed at very high pH values of approximately 13 until 15 min. However, the pH values decreased to 7.6 and 9.0 at 25 min for 0.75 and 0.9 M NaOH, respectively. A wide pH range during preparation is considered to result in the formation of heterogeneous Mg–Al LDH crystals. Furthermore, some precipitates initially formed at very high pH values may dissolve during
3
0 1.5
1.0 NaOH concentration / M
Fig. 2. Effects of NaOH concentration on the degrees of Mg2+ and Al3+ precipitated and the Mg/Al molar ratio in the precipitates.
Table 1 Variations in solution pH every 5 min during addition of Mg–Al solution and after agitation. (pH) After the agitation
NaOH/M
Addition time / min 0
5
10
15
20
25
0.75 0.9 1.0 1.25
13.4 13.5 13.6 13.6
13.2 13.3 13.5 13.5
13.0 13.1 13.3 13.3
12.6 12.9 13.1 13.1
10.4 12.4 12.7 12.8
7.6 9.0 10.8 12.6
7.9 9.1 11.0 12.6
agitation for 1 h at lower pH values. These two reasons probably lead to the low crystallinities of Mg–Al LDHs prepared from 0.75 and 0.9 M NaOH. The good crystallinities of CO3-type Mg–Al LDHs obtained over 1.0 M NaOH result from almost complete precipitation of Mg2+ and Al3+ in solution. However, the pH after agitation for 1.25 M NaOH was very high at 12.6. Therefore, the optimum NaOH concentration for Mg–Al LDH preparation by coprecipitation is 1.0 M. This stoichiometric addition of NaOH according to Eq. (1) results in precipitation of more than 99% of Mg2+ and Al3+ to give CO3-type Mg–Al LDH with a Mg/Al molar ratio of 3.0, equal to that of the Mg–Al solution.
110 113
018
009,012 015
006
003
Hydrotalcite
Relative Intensity
2000 cps
Effect of preparation method
(a)
(b)
(c)
(d) 10
20
30 40 50 2θ/ deg.(CuKα)
60
70
Fig. 1. XRD patterns of precipitates prepared from NaOH/Na2CO3 solutions with various NaOH concentrations: (a) 0.75, (b) 0.9, (c) 1.0, and (d) 1.25 M NaOH.
The effect of the preparation method on the particle properties of Mg–Al LDHs was examined. For preparation (i) or (iii), the Mg–Al solution was added to the NaOH/Na2CO3 solution or Na2CO3 solution, respectively, at constant pH. For preparation (ii), the NaOH/Na2CO3 solution was added to the Mg–Al solution. For preparations (i) and (ii), the Mg–Al and NaOH/Na2CO3 solutions were mixed at stoichiometric quantities, according to Eq. (1). For preparation (iii), the pH was adjusted to 11.0 to standardize the preparation conditions. This pH was chosen to coincide with the pH after agitation for preparation (i), as shown in Table 1. The variations in solution pH with time during the various preparations are presented in Fig. 3. As shown, the pH profiles of the three preparation methods are very different; thus, we examined the effect of these different routes on the particle properties of the prepared Mg–Al LDHs. The XRD patterns of the precipitates obtained using the three preparations confirmed the formation of CO3-type Mg–Al LDHs with Mg/Al molar ratios of 3.0. Although all the preparation methods resulted in the formation of CO3-type Mg–Al LDHs with
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14 the preparation (i)
12
the preparation (iii)
10
pH
8 the preparation (ii)
6 4 2 0
0
5
10 15 Addition time / min
20
25
Fig. 3. Variations in the solution pH over time during the various preparations.
the same Mg/Al molar ratios, the particle properties were found to be quite different. Fig. 4 shows SEM images of the freeze-dried samples. For preparation (i), very small particles of less than 0.1 mm in diameter were observed. Preparation (ii) produced secondary particle aggregates of 0.1–0.2 mm in diameter, which consisted of various sizes of plate-like crystals, whereas preparation (iii) produced secondary particle aggregates of 0.2–0.5 mm in diameter, with plate-like crystals of similar sizes. The particle size distributions in the suspensions resulting from the various preparations are presented in Fig. 5. The size distributions were 1–20 mm for preparation (i), 1–100 mm for (ii), and 2–20 mm for (iii), although the secondary particle sizes observed for the precipitates in the SEM images in Fig. 4 were less than 5 mm for all preparations. This finding supports the suggestion that CO3-type Mg–Al LDHs consist of secondary particle aggregates, and the different preparation methods were found to produce aggregates with different sizes. Furthermore, the LDH particles prepared by the different preparation methods exhibited different sedimentation properties. Fig. 6 shows the variations in the heights of the sediment over time for dilute suspensions. The suspensions displayed rapid sedimentation in the order (iii) > (ii) > (i). Preparation (iii) was found to produce CO3-type Mg–Al LDH particles with good solid–liquid separation properties. This finding corresponds to the very short time required for filtration of the LDH suspension in preparation (iii). To determine the origin of the different particle size distributions, particle morphologies, and sedimentation properties in the different preparation methods, the formation processes of Mg–Al LDHs were examined by characterizing the intermediate products at several steps during addition of Mg–Al solution or NaOH/Na2CO3 solution. When various volumes of Mg–Al solution (50, 100, 150, 200, and 250 mL) were added to 250 mL of the NaOH/Na2CO3 solution or 150 mL of the Na2CO3 solution in preparation (i) or (iii), respectively, CO3-type Mg–Al LDHs were precipitated with Mg/Al molar ratios of 3.0 at all volumes of the Mg–Al solution, and the degrees of Mg2+ and Al3+ precipitated were over 99%. Based on these results, CO3-type Mg–Al LDHs are likely precipitated throughout the addition time for preparations (i) and (iii) in Fig. 3. In preparation (i), Al(OH)4 is likely produced by the addition of the Mg–Al solution to the NaOH/Na2CO3 solution. Subsequently, Al(OH)4 reacts with Mg2+, resulting in the formation of Mg–Al LDH. For preparation (iii), to adjust the pH, a NaOH solution was added with the Mg–Al solution to the Na2CO3
Fig. 4. SEM images of freeze-dried samples: (a) preparation (i), (b) preparation (ii), and (c) preparation (iii).
solution. At the point where droplets of NaOH solution are added, the local pH will be very high. Owing to this high local pH, Al(OH)4 is produced from the added Mg–Al solution, and Mg–Al LDH is likely formed by reaction between Al(OH)4 and Mg2+. In contrast, some intermediate products were observed for preparation (ii). Fig. 7 shows XRD patterns for the precipitates obtained by addition of various volumes of NaOH/Na2CO3 solution to the Mg–Al solution for preparation (ii). The effect of the volume of NaOH/Na2CO3 solution on the degrees of Mg2+ and Al3+ precipitated and the Mg/Al molar ratio in the precipitate for preparation (ii) is presented in Fig. 8, and Table 2 gives the variations in solution pH with increasing volumes of NaOH/Na2CO3 solution. For 50 mL of the NaOH/Na2CO3 solution, neither Mg2+ nor Al3+ was precipitated owing to the low solution pH. For 75 mL, over 99% of Al3+ was
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15
15
15 ( b)
0 0
1 10 100 Particle size / μm
Distribution / %
5
5
0 0
( c)
10
10
Distribution / %
Distribution / %
(a)
10
5
5
0 0
1 10 100 Particle size / μm
1
10
100
Particle size / μm
Fig. 5. Particle size distribution profiles for the resultant suspensions: (a) preparation (i), (b) preparation (ii), and (c) preparation (iii).
300
200 (a) (b)
100
4.0
3.0 50
(c) 0 0
2.0
1.0
0
20
40
60 80 Time / min
100
0
100
150
0 250
200
Volume of NaOH-Na2CO3 solution / ml
120
Fig. 6. Variations in the height of sediment with time for dilute suspensions of (a) preparation (i), (b) preparation (ii), and (c) preparation (iii).
50
Mg/Al mole ratio ( ) in precipitate
5.0
100 Mg2+ ( ) and Al3+ ( ) precipitated / %
Height of sedimentation / ml
400
Fig. 8. Effects of volume of NaOH/Na2CO3 solution on the degrees of Mg2+ and Al3+ precipitated and the Mg/Al molar ratio in the precipitate for preparation (ii).
Table 2 Variations in the solution pH with increasing volume of NaOH/Na2CO3 solution.
2000 cps
Hydrotalcite
Al(OH)3 (a) (b)
Volume of NaOH–Na2CO3 solution/mL
50
75
100
150
200
(pH) 250
After the addition After the agitation
3.9 3.9
4.9 5.9
7.9 7.3
8.2 7.5
9.0 8.1
10.8 10.7
Relative Intensity
Initial pH: 2.7.
(c) (d) (e) 10
20
30 40 50 2θ/ deg.(CuKα)
60
70
Fig. 7. XRD patterns for precipitates obtained by the addition of various volumes of the NaOH/Na2CO3 solution to the Mg–Al solution for preparation (ii): (a) 75, (b) 100, (c) 150, (d) 200, and (e) 250 mL.
precipitated, whereas Mg2+ was not precipitated, which results in the formation of amorphous Al(OH)3. For 100 mL, both Mg–Al LDH and Al(OH)3 were observed in the XRD spectra, although the
degree of Mg2+ precipitated was very low. It was found that the Mg/Al LDH started to be formed at around pH 8. From the XRD spectra at over 150 mL, the precipitate was observed to contain only Mg–Al LDH. The degree of Mg2+ precipitated and the Mg/Al molar ratio in the precipitate increased with increasing volume of NaOH/Na2CO3 solution, and Mg–Al LDH with a Mg/Al molar ratio of 3.0 was precipitated with 250 mL of NaOH/Na2CO3 solution. Based on these results, in preparation (ii), Mg–Al LDH is considered to form through intermediate products. First, Al(OH)3 is likely produced by the addition of the NaOH/Na2CO3 solution to the Mg–Al solution. With increasing volumes of NaOH/Na2CO3 solution, Al(OH)4 probably forms by dissolution of Al(OH)3 owing to the high local pH caused by addition of the mixed alkaline solution. Then, Al(OH)4 reacts with Mg2+, leading to the formation
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of Mg–Al LDH. The gradual increase of pH from 2.5 to 4.5 observed in Fig. 3 for preparation (ii) is attributed to the consumption of OH by the formation of Al(OH)3. The second increase of pH from 8.0 to 11.0 is attributed to the consumption of OH by the formation of Mg–Al LDH. Based on these formation processes for Mg–Al LDHs, the different Mg–Al LDH particle properties obtained with the different preparation methods are considered as follows. The very small particles obtained using preparation (i) (Fig. 4(a)) are attributed to the generation of many Mg–Al LDH crystal nuclei owing to the very high pH, which do not show considerable growth. For preparation (ii) (Fig. 4(b)), various sizes of primary plate-like crystals are observed. In this case, the Mg–Al LDH crystal nuclei are considered to grow due to the low pH during preparation (pH 8.0–11.0, shown in Fig. 3), and this wide pH range probably results in the nonuniformity of the crystal sizes owing to variations during growth of the Mg–Al LDH crystal nuclei. In contrast, preparation (iii) produces plate-like crystals of similar sizes, as shown in Fig. 4(c). The constant pH leads to uniform growth of the Mg–Al LDH crystal nuclei. As shown in Fig. 5, the particle size distribution for preparation (i) is similar to that for preparation (iii), which is related to the similar formation processes for Mg–Al LDHs. However, preparation (ii) provides a wider distribution of particle sizes than preparations (i) and (iii). The first stage of preparation (ii) is considered to generate various sizes of aggregates of highly polymerized Al(OH)3 owing to repeated hydrolysis and olation. The transformation of this polymerized Al(OH)3 into Mg–Al LDH probably causes the wide particle size distribution. As presented in Fig. 6, sedimentation of the dilute suspension for preparation (iii) was faster than that for preparations (i) and (ii). The slow sedimentation for preparation (i) is attributed to the very small particles prepared. For preparation (ii), gelatinous Al(OH)3 is first produced as an intermediate product, likely resulting in the formation of gelatinous Mg–Al LDH, which could cause slow sedimentation. From these results, preparation (iii) is considered superior to preparations (i) and (ii) because it provides Mg–Al LDH with uniform primary particles, good sedimentation properties, and a narrow distribution of secondary particle aggregates, which are manageable. Hereafter, it will be necessary to develop preparation methods for Mg–Al LDH on the industrial scale by adapting the preparation (iii) method.
Conclusions In this study, three preparation methods with different pH profiles were examined for preparing CO3-type Mg–Al LDHs. Although all three methods provided Mg–Al LDHs with the same Mg/Al ratio, the particle properties were found to be significantly different owing to different formation processes. Overall, preparation (iii), in which a Mg–Al solution was added to a solution of Na2CO3 at constant pH, provided uniform crystalline LDH particles with a narrow distribution of secondary particle aggregates and excellent sedimentation properties. Scaling this method up to the industrial scale will allow sufficient particles to be produced for use in wastewater treatment. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jiec.2017.04.009. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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Please cite this article in press as: T. Kameda, Y. Umetsu, Effect of preparation method on particle properties of carbonate-type magnesium– aluminum layered double hydroxides, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.04.009